Various optical thin films are used so as to control light in the optical fields such as information communication, information recording, image pickup, and image display. Oxide thin films such as SiO2 and TiO2 and fluoride thin films such as MgF2 are used as typical optical thin film materials, and these are utilized for antireflection films, mirrors, filters, and others. Moreover, diffractive optical elements to which optical thin films are applied are used for various applications.
The diffractive optical elements are mainly classified into a relief type and a refractive index modulated type. The relief-type diffractive optical element is formed by alternately arranging local regions of a relatively large thickness and local regions of a relatively small thickness. In other words, a diffraction phenomenon appears because of a phase difference in lights which is caused by a difference in light paths between light passing through a medium of a convex portion corresponding to the local region of the relatively large thickness and light passing through the air in a concave portion corresponding to the local region of the relatively small thickness. Such a relief-type diffractive optical element can be formed by performing processes such as photolithography and etching on a surface of an optically transparent material such as quartz.
The refractive index modulated-type diffractive optical element is formed by alternately arranging local regions of a relatively large refractive index and local regions of a relatively small refractive index. A diffraction phenomenon appears because of a phase difference caused by a difference in light paths between lights that respectively pass through the local regions having different refractive indices. The refractive index modulated-type diffractive optical element can be formed by irradiating a material such as a Ge-doped silica glass or a photopolymer with an energy beam such as an ultraviolet ray or visible light so as to change (modulate) the refractive index of the material. Recently, an example of a refractive index modulated-type diffractive optical element utilizing a transparent diamond-like carbon (DLC) film has been proposed as disclosed in Patent Document 1 of Japanese Patent Laying-Open No. 2004-163892.
In comparison with the relief-type diffractive optical element that requires fine concavities and convexities to be formed, the refractive index modulated-type diffractive optical element has advantages in that its fabrication process is relatively simple and its flat surface prevents contaminants from adhereing thereto, and other advantages. However, it is difficult to greatly change the refractive index of conventional optical materials, and for example, a Ge-doped silica glass has a possible refractive index change Δn as small as approximately 0.001, and photopolymers also have a possible refractive index change Δn as small as approximately 0.08.
Here, an amount Δn of refractive index change in a refractive index modulated-type diffractive optical element directly gives influence on the diffraction efficiency. That is, the diffraction efficiency can be enhanced when the amount Δn is larger. In comparison with the relief-type diffractive optical element, therefore, the refractive index modulated-type diffractive optical element is greatly limited in designing thereof in the case of using an optical material having a small Δn.
In this respect, Patent Document 1 discloses a transparent DLC film in which Δn can be increased up to 0.5, thereby enhancing flexibility in designing the optical device. The refractive index of the DLC film can be increased by energy beam irradiation. As the energy beam, it is possible to use a corpuscular beam such as an ion beam, an electron beam, or a neutron beam, or an electromagnetic wave such as an ultraviolet ray, an X-ray, or a gamma ray. Using an ultraviolet ray among these energy beams is most preferable from the viewpoint of throughput, ease in handling, device cost, and the like when industrial application is contemplated.    Patent Document 1: Japanese Patent Laying-Open No. 2004-163892    Patent Document 2: Japanese Patent Laying-Open No. 7-333404    Patent Document 3: Japanese Patent Laying-Open No. 2004-341541    Patent Document 4: Japanese Patent Laying-Open No. 8-72193    Patent Document 5: Japanese Patent Laying-Open No. 2005-202356    Patent Document 6: Japanese Patent Laying-Open No. 11-345419    Patent Document 7: Japanese Patent Laying-Open No. 2006-39303    Patent Document 8: Japanese Patent Laying-Open No. 2005-195919    Patent Document 9: Japanese Patent Laying-Open No. 8-313845    Patent Document 10: Japanese Patent Laying-Open No. 2005-326666    Patent Document 11: Japanese Patent Laying-Open No. 10-96807    Patent Document 12: Japanese Patent Laying-Open No. 2000-235179    Patent Document 13: Pamphlet of WO2005/088364    Patent Document 14: Japanese Patent Laying-Open No. 2003-66324    Patent Document 15: Japanese Patent Laying-Open No. 2006-53992    Patent Document 16: Japanese Patent Laying-Open No. 6-27398    Patent Document 17: Japanese Patent Laying-Open No. 2006-30840    Non-Patent Document 1: “Technique of Ultraprecision Machining and Mass Production of Microlens (Array)” published by TECHNICAL INFORMATION INSTITUTE CO., LTD., Apr. 28, 2003, pp. 71-81.    Non-Patent Document 2: OPTRONICS, (2001), No. 11, pp. 149-154    Non-Patent Document 3: O plus E, Vol. 25, No. 4, 2003, pp. 385-390    Non-Patent Document 4: OPTRONICS, (2001), No. 11, pp. 143-148    Non-Patent Document 5: Large Area Display edited by Nobuo Nishida, KYORITSU SHUPPAN CO., LTD., published in 2002    Non-Patent Document 6: Applied Optics, Vol. 41, 2002, pp. 3558-3566    Non-Patent Document 7: ITE Technical Report, Vol. 20, 1996, pp. 69-72